Rotor stage of axial turbine with coak locking the vane roots

ABSTRACT

Rotor stage of an axial turbine for the gas or steam expansion comprising a shaft, adapted to rotate around a main axis, and a plurality of vanes, the shaft having an annular recess extending around the main axis, said shaft further comprising an introduction slot extending laterally from the annular recess, the annular recess being adapted to engage the roots of the vanes and the introduction slot being adapted to allow the insertion of the roots of the vanes into the annular recess, said rotor stage further comprising a coak adapted to be inserted into the introduction slot through an exclusively radial movement, to constrain the roots of the vanes in an operative position inside the annular recess.

The present invention relates to a rotor stage of an axial turbine, according to the preamble of the main claim.

The rotor stage of a gas or steam turbine comprises a shaft adapted to rotate around a main axis, on which a plurality of vanes is arranged. The vanes comprise a root and a blade, which protrudes in a substantially radial direction. The shaft has an annular recess adapted to engage the vane roots, preventing them from a movement in radial and axial direction to counter the centrifugal force and the fluid force which would pull the vane roots downstream.

In the easiest instance, the vane roots have a substantially upside-down “T” shape with two teeth extending in opposite ways along the axial direction. The annular recess, adapted to accommodate the vane roots, also has an upside-down “T” shape with two opposed undercuts adapted to accommodate the teeth of the roots.

In some implementations, the introduction of the vane root into the annular recess occurs by arranging the vane so that the root teeth are tangentially aligned to the opening of the annular recess on a plane perpendicular to the main axis. The root is inserted inside the annular recess and the vane is rotated 90° around the radial direction, so that the vane is arranged in an operative position with the root teeth inserted into the undercuts of the annular recess.

This implementation requires realizing roots having lateral size not larger than the opening of the annular recess. In addition, the insertion of the last vane needs that the roots already arranged inside the annular recess, leave an adequate space to insert and rotate the root of the last vane. This implies a certain clearance among the vanes, the spacers being introduced for the removal thereof.

A problem with this solution is that the centrifugal force acting on the vanes is very high and the roots are less thick and thus less resistant than they could be if there was not a clearance among the vanes for the need of rotating the last vane too.

In the implementation adopted by McGinnis, there is an introduction sector on the annular recess in which the two undercuts of the annular recess are cut off so that the vane roots can be introduced into the annular recess, without the need of rotating them around the radial direction. The introduction sector further has two fastening half-slots and the root of the last vane, named closing vane, has two more fastening half-slots. When all the vanes but the last one have been introduced into the annular recess, the closing vane is positioned with the root inside the annular recess in the introduction sector. The two fastening half-slots of the root are next to the two fastening half-slots of the shaft, forming two fastening slots adapted to accommodate two grab screws for fastening the closing vane to the shaft. The grab screws have an axially symmetric body with a plurality of annular protuberances. The fastening slots have annular widenings adapted to receive said annular protuberances. The body is divided in two parts on a plane comprising the symmetry axis. Both the two parts of the grab screw (half-grab screw) are positioned in one of the fastening half-slots of the shaft and the root of the closing vane. Once the coak is arranged inside the introduction slot, so that the half-grab screws match, the grab screws obtained by two half-grab screws are rotated 90° to fasten the vane to the shaft. Generally, the grab screw has a head which is riveted in order to prevent the grab screw from being rotated by vibrations inside the fastening slot during the operation.

A problem with the McGinnis implementation is that the last vane has to be realized in a different way from the others, with increasing costs.

One more problem is that the plastic deformation of the grab screw head due to the riveting and the plasticization of the grab screw body itself, because of centrifugal forces, can entail breaking the grab screw when the vane has to be disassembled. This can result in damaging the root of the closing vane and the replacement of the whole closing vane.

In a third implementation according to U.S. Pat. No. 6,135,717 on a side alongside the annular recess, an introduction slot is present that allows the introduction of the vane roots into the annular recess, without the need of their rotation. The vane roots are introduced into the annular recess through the introduction slot. The root of the last vane is locked in an operative position by a first coak adapted to catch the root teeth and by a second coak adapted to expand into the introduction slot thanks to two wedges forcefully introduced into the respective openings and locked by two screws. This implementation allows having vanes one equal to another and not damaging the vane in case the wedges have to be disposed of in the disassembling step.

A problem with this implementation is that the removal of the wedges is quite difficult. Another problem is that the locking of the last vane in an operative position requires a high number of pieces. A further problem is that the operation of forcefully introducing the wedges is quite complicated, since the wedges have to significantly deform the second coak, so that to prevent it from exiting due to centrifugal forces. The required tight working tolerances can also be a problem with this implementation.

Another problem is that the first coak has to be introduced into the introduction slot through a radial movement, at first, and then an axial movement. This requires the shaft in an axial direction having a size adequate to accommodate an introduction slot that allows the first coak such an axial movement. This entails the rotor stage having an axial size larger than other design solutions and the turbine, comprising several rotor stages, consequently resulting longer in an axial direction.

In U.S. Pat. No. 672,838 A a vane is described with an introduction slot and a coak adapted to lock the root of the last vane when said coak is introduced into the introduction slot through an exclusively radial movement, said coak being adapted to be connected to the shaft by a screw.

Object of the present invention is therefore to realize a rotor stage of an axial turbine which allows overcoming the highlighted problems.

In particular, an object is to obtain a rotor stage which does not need additional spacers among the vane roots, so that the roots can have a resistant section which is the maximum possible.

Another object is to limit the number of pieces composing the rotor stage, the number of vanes being the same.

A further object is to simplify the assembling.

Still an object is to limit costs by having all the same vanes.

Another object is to have a rotor stage having smaller axial dimensions with respect to some of the current design solutions.

Said objects are obtained by a rotor stage of an axial turbine whose inventive features are highlighted by the claims.

The invention will be better understood by the following specification; provided for illustration purposes only, thus without limitation, of a preferred embodiment illustrated in the accompanying drawings, in which:

FIG. 1 shows a rotor stage with the main axis and a plurality of vanes;

FIG. 2 shows a perspective view of a vane with the closing coak and the fastening element;

FIG. 3 shows a view from radial direction of the vane arranged in an operative position with the coak and the fastening element.

FIG. 4 relates to a longitudinal section denoted with I-I in FIG. 3, in which the shaft, the root of the vane inserted into the annular recess, the coak and the fastening element are visible;

FIGS. 5A, 5B, 5C, 5D and 5E show the assembling sequence of the last vane;

FIGS. 6A and 6B show a fastening element, in this case a pawl, and a corresponding fastening slot.

The invention relates to the rotor stage of an axial turbine for gas or steam expansion. Referring to FIGS. 1 and 2, it can be seen the rotor stage comprising a shaft 1 adapted to rotate around a main axis A, and a plurality of vanes 2, each of which has a root 3 and a blade 6, the latter projecting from the top of the root 3 in a substantially radial direction R. The blade can end with an apex or a strap. Figures relate to vanes with root whose section with a plane perpendicular to the radial direction R has a lozenged shape, but the invention also relates to vanes of different type, for example vanes with root whose section with a plane perpendicular to the radial direction R, is rectangular.

Considering the axis A of the turbine and a radial direction R, the tangential direction is perpendicular to the plane comprising the axis A and said radial direction R.

The root 3 has an upside-down “T” shape, with a first and a second tooth 4 and 5 extending in opposite ways in an axial direction. The two teeth can also extend both in axial direction and radial direction, as in the case with lozenged section represented in FIG. 3.

The root 3 can have two stiffening protuberances 19 extending in opposite ways in an axial direction. Such stiffening protuberances 19 serve to give the vane higher flexural stiffness.

The shaft 1 has an annular recess 7 extending around the main axis A and has an upside-down “T” shape, forming a first and a second opposed undercuts 9 and 10 adapted to accommodate the first and the second tooth 4 and 5, respectively, engaging the roots 3 of the vanes 2 into the annular recess 7.

Referring to FIGS. 5, the shaft 1 can be seen as also having an introduction slot 8 extending laterally from the annular recess 7 and serving for the introduction of the roots 3 of the vanes 2 into the annular recess 7.

The second undercut 10 of the annular recess 7 is in fact cut off at the introduction slot 8. The roots of the vanes are inserted one by one into the introduction slot 8 through radial movement, they are displaced into the annular recess 7 through a movement having an axial component and let slide in a tangential direction inside the annular recess 7 with the two opposed teeth 4 and 5 inserted into the two opposed undercuts 9 and 10, engaging the roots 3 of the vanes in the annular recess 7.

On the insertion of the last vane, the roots of the other vanes occupy the whole annular recess except for the area wherein the second undercut 10 is cut off and from which the introduction slot 8 separates. The root of the last vane is thus inserted into the introduction slot 8 and displaced in the annular recess 7, engaging the first tooth 4 only in the first undercut 9.

In order to constrain also the root 3 of the last vane in an operative position in the annular recess 7, the rotor stage comprises a coak 11 adapted to be inserted in an operative position into the introduction slot 8 with an exclusively radial movement. Said coak comprises a main body 20 and a catching protuberance 21 so that to have, when arranged in an operative position in the introduction slot (8), a substantially “L”-shaped longitudinal section.

When the coak 11 is inserted into the introduction slot 8 in an operative position, the catching protuberance 21 forms the extension of the second undercut 10 adapted to accommodate the second tooth (5) of the root (3), preventing the same from the radial movement.

Such a coak 11 is adapted on the whole to prevent the vane root, which is next to the introduction slot, from a radial and axial movement.

The shaft 1 has a first fastening half-slot 12A and the coak 11 has a second fastening half-slot 12B in such a position that when the coak 11 is inserted into the introduction slot 8, the two fastening half-slots 12A and 12B are facing one to, another forming a fastening slot 12 partially obtained in the shaft 1 and partially in the coak 11.

The rotor stage further comprises a fastening element 13 adapted to be introduced into said fastening slot 12 and to lock the coak 11 in the introduction slot 8, the coak 11 in its turn locking the root of the corresponding vane inside the annular recess. The fastening slot 12 has at least one widening 17 closer to the main axis A and a narrowing farther from the main axis A, that forms a bottleneck or neck 18.

The fastening element 13 has a protuberance 16 adapted to be inserted into the widening 17 of the fastening slot 12 and large enough to not allow the passage thereof through the neck 18. The fastening element 13 can comprise a rivet inserted into the fastening slot 12 through the narrower neck and then expanded inside the widening.

In the preferred embodiment the fastening slot 12 has an axially symmetric shape with annular widenings 17 alternating with annular narrowings which form as many necks 18.

The fastening element 13 preferably comprises a head 14 and a body 15. The body 15 has an axially symmetric shape around a symmetry axis and is adapted to be inserted into the fastening slot 12. The fastening element 13 can rotate inside the fastening slot 12.

The fastening element 13 is divided in two separated parts, named first and second fastening half-elements 13A and 13B, by a plane crossing the body 15 and comprising the symmetry axis of the body 15 itself.

The fastening element 13 comprises one or more annular protuberances 16 on the body, which are adapted to be inserted into the annular widenings 17 inside the fastening slot 12 and locked by the necks 18 of the fastening slot 12, so that to prevent the fastening element 13 from sliding out in a radial direction R.

When the coak 11 is arranged inside the introduction slot 8 with the two parts 13A and 13B of the fastening element 13 in the two fastening half-slots 12A and 12B, in order to lock the fastening element 13 in the fastening slot 12, the fastening element 13 is rotated around its own symmetry axis so that the two parts 13A and 13B, which compose the same, are arranged transversely the two half-slots forming the fastening slot 13, thereby preventing the coak 11 from a movement in a radial direction R.

In order to obtain a larger reduction of the axial dimension of the rotor stage, one or two fastening slots 12 can be present and arranged laterally to the coak 11, so that a straight line, departing in a tangential direction T from any point of the fastening slot 12, intersects the coak 11.

The assembling of the vanes on the shaft is represented in FIG. 5.

Referring to FIG. 5A, the root 3 of the vane 2 is introduced into the introduction slot 8 through a movement in a radial direction.

In FIG. 5B the vane 2 can be seen as being displaced in an axial direction by entering the root 3 of the vane 2 in the annular recess so that the first tooth 4 is inserted into the first undercut 9.

The roots 3 of the vanes 2 are then slid inside the annular recess 7.

The root of the last vane remains next to the introduction slot since the roots of the other vanes completely take up the rest of the annular slot.

In FIG. 5C, the first fastening half-element 13A is seen as being inserted into the first fastening half-slot 12A present on the shaft 1.

In FIG. 5D, the second fastening half-element 13B is seen as being inserted into the second fastening half-slot 12B present on the coak 11.

In FIG. 5E, the coak 11 with the second fastening half-element 13B is seen as being inserted into the introduction slot 8 through a radial movement, so that the two fastening half-slots are facing to form a fastening slot partially obtained in the shaft 1 and partially in the coak 11. In this way the first and the second fastening half-elements 13A and 13B are also facing to form a fastening element 13 inserted into the fastening slot 12.

The fastening element 13 is then rotated 90° so that to prevent the coak 11 from being drawn out from the introduction slot 8.

The head of the fastening element is riveted such that it acquires a plastic deformation, so that the fastening element 13 can not spontaneously rotate during the operation, for example due to vibrations.

In the disassembling step, the fastening element 13 is forcefully rotated so that to allow the coak 11 to be drawn out from the introduction slot 8 and the vanes from the annular recess.

Such an operation can result in damaging or destroying the fastening element 13 and also damaging the coak 11, which in this case will have to be replaced. Such an operation can not conversely cause the damage of the vane 2, which is apart from the fastening element 13, on which operation is needed to release the coak 11 and the vanes 2.

The fastening element can be a grab screw, as in FIG. 2, with only one body provided with annular protuberances and divided in two parts, or else it can be a pawl, such as in FIGS. 6A and 6B with a body 15 divided in two parts and provided with annular protuberances and a head 14 made in a single piece.

The fastening element could also be a rivet, which has to be destroyed in the disassembling step.

The fastening slot 12 can be divided in two equal half-slots or also in two differently sized half-slots.

The vane root could also have only one tooth and the annular recess could have only one undercut, the coak 11 having in this case a substantially cylinder shape, for example a parallelepiped (rectangular-directrix cylinder) with a main body and without catching protuberance.

A first advantage of the rotor stage according to the invention is to be constituted by a limited number of pieces and to allow an easy assembling and disassembling of the rotor vanes.

A second advantage is that the coak 11 being adapted to be inserted in an operative position through an exclusively radial movement, the rotor stage is shorter in an axial direction and the turbine firmer.

A third advantage is that all the vanes are made in the same way, with no different closing vane. The closing vane can possibly have a partial or total removal of one of the stiffening protuberances 19.

A fourth advantage is that the roots of the vanes according to the invention do not need a clearance among them and can thus have larger size so that to be more resistant.

A fifth advantage according to the invention is that in the disassembling step the vane is not damaged, also in case the fastening element 13 has to be destroyed. 

1. Rotor stage of an axial turbine for gas or steam expansion comprising a shaft, adapted to rotate around a main axis, and a plurality of vanes, each of these having a root with at least one first tooth protruding in an axial direction, and a blade protruding from the top of the root in a substantially radial direction, the shaft having an annular recess extending around the main axis and comprising at least one first undercut adapted to accommodate the first tooth, said shaft further comprising an introduction slot extending laterally from the annular recess, the annular recess being adapted to engage the roots of the vanes and the introduction slot being adapted to allow the insertion of the roots of the vanes into the annular recess, said rotor stage further comprising a coak adapted to be inserted into the introduction slot to constrain the roots of the vanes in an operative position inside the annular recess, said coak being adapted to be inserted in an operative position into the introduction slot through an exclusively radial movement, characterized in that the shaft has a first fastening half-slot and the coak has a second fastening half-slot positioned such that, when the coak is inserted into the introduction slot, the two fastening half-slots and are facing one to another forming a fastening slot partially obtained in the shaft and partially in the coak, said rotor stage further comprising a fastening element adapted to be introduced inside said fastening slot and to lock the coak in the introduction slot.
 2. Rotor stage of an axial turbine according to claim 1, wherein the root of each vane comprises a second tooth departing axially and in a way opposite to the first tooth, the annular recess having a second undercut opposed to the first undercut, both the two undercuts being adapted to receive one between the first and the second tooth, the introduction slot cutting off the second undercut, said coak comprising a main body and a catching protuberance that, when the coak is arranged inside the introduction slot in an operative position, completes the second undercut so that to accommodate the second tooth of the root, preventing the radial movement thereof.
 3. Rotor stage of an axial turbine according to claim 1, wherein said coak arranged in an operative position inside the introduction slot, has a substantially “L”-shaped longitudinal section.
 4. Rotor stage according to claim 1, wherein characterized in that said fastening element comprises at least two separated parts.
 5. Rotor stage according to claim 1, wherein said fastening slot has an axially symmetric shape, with at least one annular widening and a neck, so that the fastening element can rotate inside the fastening slot.
 6. Rotor stage according to claim 5, wherein said fastening element comprises at least one body with at least one protuberance.
 7. Rotor stage according to claim 6, wherein said protuberance of the fastening element is an annular protuberance of axially symmetric shape, said fastening element being adapted to rotate around its own axis when inserted into the fastening slot.
 8. Rotor stage according to claim 7, wherein the body of the fastening element is divided in two parts along a plane comprising the axis thereof.
 9. Rotor stage of an axial turbine according to claim 1, wherein the roots of the vanes have a section with a plane perpendicular to the radial direction of a substantially lozenged shape. 